Cellular radiotelephone systems are increasingly being used for wireless mobile communications. As known to those having skill in the art, a cellular radiotelephone system is a wide area communications network which utilizes a frequency reuse pattern in a plurality of cells. The design and operation of an analog cellular phone system are described in an article entitled "Advanced Mobile Phone Service" by Blecher, IEEE Transactions on Vehicular Technology, Vol. VT 29, No. 2, May 1980, pp. 238-244. The analog mobile cellular system is also referred to as the "AMPS" system.
Recently, digital cellular phone systems have also been proposed and implemented using a Time-Division Multiple Access (TDMA) architecture. Standards have also been set by the Electronics Industries Association (EIA) and the Telecommunications Industries Association (TIA) for an American Digital Cellular (ADC) architecture which is a dual mode analog and digital system following EIA/TIA standard IS-54B. Telephones which implement the IS-54B dual mode architecture are presently being marketed by the assignee of the present invention. Different standards have been promulgated for digital cellular phone systems in Europe. The European digital cellular system, referred to as GSM, also uses a TDMA architecture.
A cellular radiotelephone network includes a plurality of spaced-apart small radio zones called cells. Each cell includes a base station for transmitting and receiving messages to and from cellular radiotelephones which are located within the cell range. A plurality of base stations are connected to a Mobile Telephone Switching Office (MTSO) which acts as a controller for the cellular system. The MTSO is also connected to the Public Switched Telephone Network (PSTN) to allow communications between cellular radiotelephones and wire line phones.
As used herein, the term "cellular radiotelephone" encompasses a wide variety of portable radiotelephone devices which access a cellular radiotelephone system. Cellular radiotelephones include portable telephones of the hand-held or bag phone variety and permanently mounted car cellular telephones. The term "cellular radiotelephone" also includes terminals which provide functions in addition to those of a cellular telephone, such as facsimile, data communications, data processing, word processing applications and other personal communications systems functions. These high function cellular radiotelephones are often referred to as "Personal Communications Systems" (PCS).
Each cell of a cellular radiotelephone system typically includes a plurality of duplex voice channels over which cellular radiotelephone messages are carried. Each cell is also provided with a plurality of control channels to control the operation of the cellular radiotelephones and to assign voice channels. There are three basic types of control channels in the cellular radiotelephone systems described by EIA/TIA Standard IS-54B and other conventional cellular system standards. They are Dedicated Control Channels, Paging Channels and Access Channels. The cellular frequency band is typically divided into 832 channels of which 416 are allocated to a first carrier, known as the "A Carrier" and 416 are allocated to a second carrier, known as the "B carrier". Each carrier has its own set of channels that are distinct and non-overlapping in a given geographic area. Thus, it is ensured that upon initial access, a cellular radiotelephone will access its home system (either the A Carrier or the B Carrier).
Of the 416 channels allocated to each carrier, 21 channels are set aside to be used as Dedicated Control Channels. The Dedicated Control Channels are defined by the standard to occupy a specific limited channel range in the cellular network. These Dedicated Control Channels are used by a cellular radiotelephone initially to find service on the system of a desired type, either A or B. Since the channel numbers and therefore frequency of the Dedicated Control Channels are known in advance, the cellular radiotelephone can scan these channels to determine if service is available.
A plurality of Paging Channels are also provided. Paging Channels are channels to which cellular radiotelephones tune to and wait for page or other control messages. A plurality of Access Channels are also provided. Access Channels are channels to which a cellular radiotelephone sends responses to page and other control messages, or sends an origination request in response to a user request to place a telephone call.
A cellular carrier can define Access Channels to be separate from Paging Channels, and place the Access Channels in any valid range for the cellular system type. Separate Paging and Access Channels are typically not used, however, because of the complexity involved in setting up a cellular radiotelephone system in what is called split paging and access. Accordingly, most systems combine Paging and Access Channels to provide what will be referred to herein as "Combined Paging and Access Channels". In systems that use Combined Paging and Access Channels, a bit in the overhead message data (CPA) informs the cellular radiotelephone that the Paging and Access Channels are the same.
When a cellular radiotelephone is powered on, it performs an initialization procedure with the cellular radiotelephone system. In general, the cellular telephone first scans a stored range or set of Dedicated Control Channels which are assigned, one per cell, and are used by the cellular radiotelephone system to convey digital control information between the MTSO and the cellular radiotelephone. A multiple bit System IDentification (SID) is transmitted as part of the digital overhead information from the MTSO on the control channel. After this initial scan, the cellular radiotelephone selects the Dedicated Control Channel having the highest signal strength and stores the system identification of this Dedicated Control Channel in its memory. The channels of the Paging Channel set are then scanned for the strongest channels, and their identity is saved.
When scanning the Paging Channels, the SID of the Paging Channel and Access Channel is compared to determine whether an acceptable SID is present. It will be understood by those having skill in the art that an acceptable SID may be limited to the same SID as the Home SID or may broadly include a particular group of SIDs for which arrangements have been made by the home carrier, or any other combination of selected criteria.
After the above-described initialization procedure is complete, the cellular radiotelephone enters an "idle task" and awaits an indication that a system access is required. A system access may be required in response to an indication from the MTSO that the cellular radiotelephone is being called, also referred to as a "page". Other control messages which require a cellular radiotelephone response may also be received. Alternatively, the user of the cellular radiotelephone may attempt to place a telephone call, and thereby require access to the cellular radiotelephone system.
A major problem occurs when a system access is attempted by a cellular radiotelephone, and the cellular radiotelephone is in a geographic area of overlap between adjacent cellular radiotelephone systems. In this situation, the cellular radiotelephone may access the adjacent system when the cellular radiotelephone wishes only to access the "home" system. Since the adjacent system did not originate the telephone call to the cellular radiotelephone, and the cellular telephone is not registered to place telephone calls in the adjacent cellular system, the system access will fail. Alternatively, system access may occur via the adjacent system, thus penalizing the cellular telephone with roaming charges. Both of these situations are undesirable.
A known solution which prevents undesired access to an adjacent system is described in U.S. Pat. No. 4,905,301 to Krolopp et al. entitled "Selective System Scan for Multizone Radiotelephone Subscriber Units". According to the Krolopp et al. patent, an attempt to access an undesired radiotelephone system is prevented by comparing the SID transmitted on a Dedicated Control Channel of the desired radiotelephone system and the SID transmitted on an Access Channel during the access process. Access is permitted if the Dedicated Control Channel SID and the Access Channel SID match.
The above-described solution requires a comparison of SIDs between the Dedicated Control Channel and the Access Channel prior to permitting system access. There is a need to provide methods and systems which allow proper accessing of a cellular radiotelephone system by a cellular radiotelephone without requiring an SID comparison during access.